In addition to Zaxos correct idiom it is useful to note you can use do{} in a ternary op which lets you say do:

$condition ? do{ $a = $b } : do { $c = $d };
$condition ? do_stuff() : do{ $stuff = NOT_DONE };
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ie mix assignments to different vars, assignment and function call etc. This is mostly useful if you are using it as a shortform idiom for if/elseif..... If you are just assigning different values to a variable based on some condition Zaxos $var = $cond ? $val1 : $val2; is the way to go.